Belknap Mountain stands within the watershed of the Merrimack River, which drains into the Gulf of Maine in Massachusetts.

[3] This series of mountains of igneous origin represents volcanic activity that occurred during the Jurassic period, about 200-150 mya.

The White Mountain Magma series is thought to have formed above an aborted transform fault in the crust.

The Belknap Mountains as observed today are the remnants of erosion of those volcanics and the metamorphic rocks underneath them, together representing about 4 miles (6 km) in depth.

As described in detail by Long,[3] contemporary features of the Belknap Mountains include the former magma chamber, and inner and outer ring dike formations that surround collapsed blocks of Devonian rocks and moat volcanics.